My copending application Ser. No. 903,560, filed Sept. 8, 1986, discloses a feed box having a discharge spout of which one jaw or side is fixed and the other has a flexibly hinged lower part, and adjusting means on the box and acting on the hinged lower part for controlling the rate of feed from the box by varying the cross-sectional area of the spout's outlet. Following common practice, as in Leeman U.S. Pat. No. 3,344,919, the feed control of my copending application is illustrated applied to a sieve, screen or sieve bend mounted in a housing or holder having counterpart ends and reversible end-for-end for alternating the ends of the screen presented for feeding. The present invention is an improvement on that of my copending application.